In an ongoing survey by the NIDA, marijuana is the most commonly used illegal drug. Nearly 17 million Americans ages 12 or older reported that they had used marijuana at least once in the month before the survey.

The main active ingredient in marijuana is a chemical called THC. This chemical is responsible for the intoxicating effect. When a person smokes marijuana, the THC is rapidly absorbed from the lungs into the bloodstream. In the bloodstream, it is carried to the brain and other organs. In the brain, it disrupts the normal process of natural chemicals and leads to intoxication. THC is similar to some of those natural chemicals found in the body. The chemicals that THC affects play a role in memory, concentration, perception, coordination, pain sensation, immunity to disease, and reproduction. Marijuana also contains more than 400 other chemicals, including cancer-causing agents. Marijuana also contains more than 400 other chemicals, including cancer-causing agents. Marijuana smoke contains up to 70 percent more cancer agents than tobacco smoke. Marijuana users usually inhale more deeply and hold their breath longer than tobacco smokers do, which increases the lungs' exposure to the smoke.

When marijuana is smoked, its effects begin immediately after the drug enters the brain and lasts from one to three hours. THC causes brain cells to release a chemical that produces a feeling of pleasure. THC also probably alters the way the brain processes information. This may be the cause of the short-term memory loss. Research findings for long-term marijuana use indicate some changes in the brain similar to those seen after long-term use of other major drugs.

The marijuana available today can be five times more potent than the marijuana of the 1970s. Marijuana's effect depends on the strength of the THC it contains. Most ordinary marijuana contains about 3 percent THC. Sinsemilla, which is made from just the buds and flowering tops of female marijuana plants, is about 7.5 percent THC, but can range as high as 24 percent. Hashish, made from the sticky resin from the female marijuana plant flowers, is about 3.6 percent THC, but can range as high as 28 percent. Hash oil, a tar-like liquid distilled from hashish, is about 16 percent THC, but can range as high as 43 percent.

When the early effects fade after a few hours, users become very sleepy. Other signs of use include depression, fatigue, or hostility. Long-term effects include deteriorating relationships with family members and friends, a drop in grades, increased absenteeism or truancy, lost interest in sports or other favorite activities, and changes in eating or sleeping habits.

Other popular street terms for marijuana include weed, widow, herb, pot, and grass. Most people who use marijuana roll the loose plant parts into a cigarette, which is called a joint or nail, or smoke it in a pipe, which may be called a bong. Some users cut open a cigar, remove the tobacco, and replace it with marijuana and often crack cocaine. These may be called blunts, primos, or woolies. A marijuana-laced cigar may also be dipped in another drug called PCP; this may be called a happy stick, wicky stick, or love boat.

Marijuana has been called a "gateway" drug, because it’s often the first drug a person uses before moving on to other illegal substances. For instance, although few youths use cocaine, the risk of trying it is greater for a young person who has tried marijuana than for someone who has not. Some research indicates that long-term marijuana use may alter the brain, making it easier to become addicted to alcohol, cocaine, or other drugs.